Project Summary For neural networks to be refined and shaped by experience, the expression of new genes in response to activity is critical. Neural activity triggers the expression of immediate early genes (IEGs) such as arc, fos, and egr1 within minutes, but many IEGs are transcription factors that in turn trigger subsequent waves of transcription. These later waves of transcription are necessary for the consolidation of stable, persistent memories, yet identity and function of the genes in these later waves remains unknown. Further, it is unknown if all activated neurons in a circuit are identically modulated, or if cell-specific transcriptional changes can drive emerging functional differences. The extent of this heterogeneity at the level of the individual neuron has been completely unexplored. Only now, with recent advances in single-cell sequencing technologies, is it possible to track gene expression changes in individual activated neurons. This project will track activity-related gene changes in single neurons of the hippocampal dentate gyrus, a region critical for learning and memory, providing for the first time a transcriptional `signature' of the activity in individual neurons. This will be achieved by examining dentate granule cells in three aims; First, the long-term waves of transcription will be characterized during the full time span during which transcription and translation inhibitors have been demonstrated to impair memory. Second, the impact of each transcriptional wave will be explored in terms of changes in cell excitability and the probability of activation to a subsequent event either in vivo or in vitro. Finally, the dentate gyrus is one of the few brain regions that incorporates new neurons during adulthood in mammals. Immature dentate granule cells are highly similar to their mature counterparts, except that they possess distinct electrophysiological properties which help to control function across the entire dentate gyrus. Therefore activity-related transcription of this unique population of immature cells will be characterized, as well as their functional and transcriptional impact on mature cells. Findings from this project will reveal novel links between gene expression and function in individual neurons, identifying novel targets for the maintenance of memory in healthy humans and in restoring memory function in aging or disease states.